1. FIELD OF THE INVENTION
This invention relates to a pressure image forming apparatus which fixes, under pressure, an image obtained by exposure to an original image.
2. DESCRIPTION OF THE PRIOR ART
An example of an image recording medium in which microcapsules containing a photosensitive composition are utilized is disclosed in Japanese Kohai No. 179836/1982. This system has a substrate which carries capsules made of a synthetic polymeric resin and containing a vinyl compound, a photopolymerization initiator and a coloring precursor.
In an image recording method using this system, microcapsules are hardened in correspondence with an original image by being exposed thereto, and parts of the microcapsules which have not been hardened are ruptured by pressing so as to release the coloring agent precursor, thereby forming a color image. This method ensures that an image of a high quality can be obtained by simple dry-type processing, but this system suffers from a disadvantage in that the photosensitivity is much lower than with a system which makes use of silver halide.
The present applicant has proposed, in the specification of Japanese Kokai No. 275742/1986, a novel recording medium which has a high photosensitivity and which makes it possible to obtain an image of a high quality by a simple dry-type process. This recording medium comprises a photosensitive composition in which at least a photosensitive silver halide, a reducing agent, a polymerizable compound and a color image forming substance are applied over a substrate and in which at least the polymerizable compound and the color image forming substance consist of a photosensitive composition confined in common microcapsules.
An image recording method which makes use of this photosensitive medium is disclosed in the specification of Japanese Kokai No. 278849/1986 filed by the same applicant. According to this method, the photosensitive medium is first exposed so that a latent image is formed in conformity with an image to be recorded. The medium is thereafter developed by heating so that the polymerizable compound in the area where the latent image exists is polymerized to produce a polymeric compound, thereby setting the microcapsules. Then, the medium is superposed on an image receiving medium having an image receiving layer capable of receiving a color image forming substance, and is pressed onto the image receiving medium so that at least part of the microcapsules existing in areas where no part of the image is formed are ruptured so as to transfer the color image forming substance to the image receiving medium, thereby forming an image on the image receiving medium. It goes without saying that, in this heat transfer process, the larger the pressing force, the higher the coloring density of the transferred image.
In a known example of a pressing device used to effect the above-described process, the photosensitive medium and the image receiving medium superposed on each other are made to pass between a pair of press rollers. However, the pressing force thereof is constant and cannot be changed. Therefore, it is not possible for this type of device to satisfy a need to change, in accordance with the type of original character or photograph, a development factor which represents the relationship between an increment in the exposure value and a change in the coloring density of the image, namely, a so-called gamma (.gamma.).